The field of the present invention relates to a magnetic resonance imaging (MRI) apparatus and a magnetic resonance imaging method and, more particularly, to a magnetic resonance imaging apparatus and a magnetic resonance imaging method in which a subject is scanned in accordance with an imaging sequence for applying an RF pulse to each of imaging slice areas in a static magnetic field space in which the imaging slice areas each include an imaging target body-moved at the subject are accommodated or held, and acquiring magnetic resonance signals produced from the imaging slice areas as imaging data, and thereafter image reconstruction processing is executed on the imaging data acquired by scanning the subject in the imaging sequence, thereby reconstructing images of the imaging slice areas.
A magnetic resonance imaging apparatus has been used in various fields such as a medical field, an industrial field, etc.
The magnetic resonance imaging apparatus includes an imaging space formed with a static magnetic field. Each imaging slice area including a target for imaging at a subject is accommodated or held in the imaging space. Thus, spins of proton in the imaging slice area are arranged in the direction of the static magnetic field to obtain magnetization vectors thereof. Thereafter, an RF pulse is transmitted to each imaging slice area of the subject in the imaging space formed with the static magnetic field to generate a nuclear magnetic resonance (NMR) phenomenon, thereby flipping the magnetization vectors of the spins. Then, magnetic resonance (MR) signals generated when the magnetization vectors of the flipped spins are returned in an original static magnetic-field direction, are acquired. Thus, the subject is scanned in accordance with an imaging sequence for imaging the imaging slice areas to acquire the magnetic resonance signals as imaging data. The subject is scanned in accordance with, for example, an imaging sequence such as a spin echo method, a gradient recalled echo method or the like. Then, image reconstruction processing is effected on magnetic resonance signals acquired as imaging data by execution of this scan to generate slice images about imaging slice areas of the subject.
There may be cases in which when the subject is scanned using the magnetic resonance imaging apparatus in this way to generate the slice images, body motion artifacts occur in the generated slice images where an imaging target is body-moved and its position is displaced from a reference position. Since each internal tissue is moved due to a breathing exercise and heartbeat motion or the like where, for example, a region such as a cardiac region of the subject, an abdominal region thereof or the like is imaged, body motion artifacts may occur noticeably and image quality may be deteriorated.
In order to prevent deterioration of the image quality due to the occurrence of the body motion artifacts, a method of monitoring the body motion has been proposed that is based on the breathing exercise or the heartbeat motion or the like, such that a scan is executed in sync with the body motion (refer to, for example, Japanese Unexamined Patent Publication No. Hei 10(1998)-277010).
Further, there has been proposed a slice tracking method for monitoring body motion at the execution of a scan, calculating the position of an imaging target body-moved from a reference position and thereafter adjusting an imaging sequence condition in such a manner that each magnetic resonance signal is acquired from its corresponding imaging slice area including the body-moved imaging target. Here, for example, the frequency of each RF pulse is adjusted from a reference frequency, based on the result of calculation of the position of the imaging target body-moved from the reference position, followed by its transmission. For instance, each receive frequency and phase are adjusted. Thus, the slice tracking method adjusts various parameters starting with the frequency of the RF pulse and sets an imaging sequence condition (refer to, for example, Japanese Unexamined Patent Publication No. 2006-26076). Patent Document 1. Japanese Unexamined Patent Publication No. Hei 10(1998)-277010.
In the case of this method, the acquired imaging data are not selected as the raw data where the diaphragm varies greatly and its position is beyond the acceptance window. When the diaphragm is small in variation, the acquired imaging data is selected as the raw data and each slice image is reconstructed. It is, therefore, possible to prevent the occurrence of body motion artifacts in each image-reconstructed slice image. In this method, however, ones unselected as the raw data exist in the acquired imaging data. Therefore, the scan time may become long and hence imaging may not be carried out efficiently.
On the other hand, in the latter method, an imaging sequence is executed following a navigator sequence after execution of the navigator sequence in a manner similar to the above in order to monitor the breathing exercise of a subject, for example. Here, the position of each imaging slice area including an imaging target, moved upon the execution of the imaging sequence is determined based on the position of the diaphragm calculated as described above by execution of the navigator sequence. Thereafter, the frequency or the like of each RF pulse transmitted in the imaging sequence is adjusted and the RF pulse subsequent to having been adjusted in frequency is transmitted, thereby scanning the subject so as to correspond to the moved imaging slice area.
Therefore, since this method yields the absence of the imaging data unselected as the raw data, the scan time becomes short as compared with the former method, thereby making it possible to execute imaging efficiently.
In the latter slice tracking method, however, when the RF pulse subsequent to having been adjusted in frequency so as to correspond to each imaging slice area displaced from the reference position due to the body motion is transmitted upon execution of the scan in the imaging sequence, the phase of the magnetic resonance signal produced in each imaging slice area moved from the reference position may be different from that of the magnetic resonance signal produced at the reference position. Thus, there may be a case in which a phase difference occurs between the two. Therefore, when the subject is scanned in accordance with the imaging sequence for transmitting the RF pulse subsequent to having been adjusted in frequency, and image reconstruction processing is executed on the acquired or collected imaging data, ghosts occur in the reconstructed images, so image quality may be deteriorated. This malfunction comes to the surface because the above phase difference becomes larger as the imaging slice area is greatly displaced from the reference position due to the body motion.
Such a malfunction further comes to the surface depending on a delay time and an error in phase at the transmission of the RF pulse, and the conditions of hardware in addition to this.
Since the ghosts occur upon execution of the imaging by the slice tracking method in this way, an improvement in image quality may become difficult.